The present invention relates to an optoelectronic device incorporating a tunable laser and a method for the characterisation thereof.
Tunable single section lasers are known which are tunable by changing the operating temperature of the laser, and methods of characterising such lasers, that is determining the operating conditions required to emit a given wavelength, are also known from, for example WO 98/00893 and U.S. Pat. No. 5,019,769. Because of variations between devices, such characterisation must be carried out for each individual device, with the operating conditions being stored in a memory associated with the laser. However, such single section lasers are tunable over only a small range of wavelengths.
Multi-section tunable semiconductor lasers are known, from, for example, EP-A-0300790, which are tunable over a wider range of wavelengths by adjusting currents injected into filter and phase sections of the laser. However, such lasers are subject to mode jumps between different wavelengths. For stable operation it is important to determine operating conditions in a mode plane away from these mode boundaries.
A method of characterising such multi-section lasers is disclosed in WO 99/40654. The position of mode planes is determined and at least a part of the mode planes are stored in a memory. However, the corresponding wavelength of the emitted radiation may only be determined by an external instrument in order to be stored in the memory to form a look-up table of operating conditions required for particular wavelength emission. This is particularly inconvenient for necessary re-calibration during use. The time to characterise each laser is also considerable.
The applicant is also aware of WO 00/52789, WO 00/54380 and WO 00/54381, which all have priority dates earlier than the priority dates of the present application, but were published after the priority dates of the present application, and therefore can form part of the state of the art for novelty purposes only.
The object of the present invention is at least partially to ameliorate the above difficulties.
According to a first aspect of the present invention there is provided an optoelectronic device including a tunable laser hybridised with wavelength measuring means for measuring wavelength of radiation emitted by the laser, wherein the wavelength measuring means includes long period diffraction grating means having a known relationship between the wavelength of an incident beam and a proportion of power of the incident laser beam transmitted by the grating.
Preferably the diffraction grating means comprises a long period diffraction grating filter.
Alternatively the diffraction grating means comprises a Bragg photonic band-gap crystal.
Preferably the hybridised optoelectronic device further includes wavelength-locking means for locking the laser onto one of a plurality of predetermined wavelengths.
Preferably, the tunable laser is a multi-section laser.
Conveniently, the wavelength measuring means further comprises: input power measuring means for measuring input power of the incident beam; output power measuring means for measuring output power of a beam transmitted by the grating and processing means to calculate the proportion of input power to output power to determine the proportion of power transmitted and hence to determine the wavelength of the transmitted beam.
Advantageously, the input power measuring means comprises input sampling means for providing a sampling beam from the beam emitted by the laser and an input photoelectric diode to measure power of the sampling beam.
Conveniently, the input sampling means comprises an input beam splitter.
Preferably, the input beam splitter comprises a refractive index discontinuity in a waveguide through which the input beam is transmitted, such that a proportion of the incident beam is deflected out of the waveguide by the discontinuity.
Conveniently, the tunable laser has a first power output facet and an opposed second monitoring output facet and the input sampling means is adapted to sample a beam output from the second monitoring output facet.
Preferably, the diffraction grating means is such that the proportion of power of the input beam transmitted by the diffraction grating means is proportional to the wavelength of the input beam.
Conveniently, the output power measuring means comprises output sampling means for sampling power transmitted by the diffraction grating means and an output photoelectric diode to measure output sampled power.
Advantageously, the output sampling means comprises an output beam splitter.
Conveniently, the output beam splitter comprises a refractive index discontinuity in a waveguide through which the output beam is transmitted, such that a proportion of an incident beam is deflected out of the waveguide by the discontinuity.
Alternatively, the output beam splitter comprises a photonic band-gap artificial crystal in a waveguide through which the output beam is transmitted, such that a proportion of an incident beam is deflected out of the waveguide by the artificial crystal.
Advantageously, the wavelength locking means comprises: a Fabry Perot or Fizeau etalon for transmitting only laser beams of predetermined wavelengths, the etalon being located in an output beam from the laser, and power measuring means for measuring the power of the output beam transmitted by the etalon and feedback means for controlling the wavelength of the beam emitted by the laser dependant on the power measured by the power measuring means.
Preferably, the tunable laser, the wavelength measuring means and the wavelength locking means are arranged in a planar array.
Conveniently, the optoelectronic device further includes one of a silicon, a silicate and a silicate-on-silicon substrate.
Advantageously, the wavelength locking means comprises a Fabry Perot etalon of glass or quartz, or a Fizeau etalon of glass or quartz or a gas-filled void.
Alternatively, the optoelectronic device is used in combination with wavelength locking means external to the device.
According to a second embodiment of the first aspect of the invention, there is provided an optoelectronic device comprising: a silicon or silicate substrate, a multi-section laser mounted on the silicon or silicate substrate, an output optical fiber optically count to the laser for transmitting a laser beam from a first power output facet of the leser, a first integrated waveguide for transmitting a monitoring beam from a second monitoring output facet of the laser to an input of a long period grating is filter, a beam splitter in the first waveguide for splitting the monitoring beam between the long period grating filter and a first photoelectric diode for measuring the power of the monitoring beam, a second integrated waveguide connected between an output of the long period grating filter and an input of a Fabry Perot or Fizeau etalon adapted to transmit predetermined frequencies, a second beam splitter in the second integrated waveguide for splitting a laser beam emergent from the long period grating filter, a second photoelectric diode for measuring the power of the beam emergent from the long period grating filter, a third photodiode for measuring power emitted from the etalon for determining when the power of the beam emitted from the etalon is a local maximum corresponding to one of the predetermined frequencies, and control means for controlling currents to sections of the laser, dependent on a signal received from the third photodiode, whereby the wavelength of a beam radiated from the laser may be determined by the filter means to produce a look-up table of laser operating conditions to produce a given wavelength and the control means may be used to lock the laser to one of the predetermined frequencies.
According to a second aspect of the invention, there is provided a beam splitter in an optical waveguide having a longitudinal axis, the beam splitter comprising an interface in the waveguide between a first portion of the waveguide having a first refractive index and a second portion having a second refractive index, wherein the interface is inclined to the longitudinal axis to reflect, or tap, out of the waveguide a proportion of optical radiation incident on the interface.
Conveniently, the optical waveguide is a planar optical waveguide.
Preferably, the interface is inclined at substantially 45 degrees to the longitudinal axis.
Advantageously, the optical waveguide is formed on a plane of a substrate and the interface is inclined to reflect a portion of the incident beam parallel to the plane of the substrate.
Conveniently, the interface taps a proportion of the incident beam into a second waveguide, preferably forming a T-junction or a Y-junction.
Alternatively, the optical waveguide is formed on a plane of a substrate and the interface is inclined to reflect a portion of the incident beam perpendicular to the plane of the substrate.
According to a third aspect of the invention, there is provided a method of forming a beam splitter in an optical waveguide having a longitudinal axis, the method comprising the step of forming an interface in the waveguide between a first portion of the waveguide having a first refractive index and a second portion having a second refractive index, wherein the interface is inclined to the longitudinal axis to reflect a proportion of optical radiation incident on the interface out of the waveguide.
Conveniently, the step of forming an interface comprises the steps of providing a region of photosensitive material within the first portion of the waveguide and exposing the photosensitive material to ultraviolet light to from the second portion.
Alternatively, the step of forming an interface comprises the steps of cutting a slot in the waveguide and filling the slot with material of a different refractive index from that of the waveguide.
Conveniently, the step of filling the slot comprises filling the slot with nitrogen.
According to a fourth aspect of the invention, there is provided a method of characterising a hybridised optoelectronic device comprising a tunable multi-section laser hybridised with wavelength measuring means, the method comprising the steps of a) providing data input/output means stepwise to increment electric currents to sections of the multi-section laser such that the laser emits laser radiation, b) using the wavelength measuring means to measure the wavelength of radiation emitted from the laser and c) storing in a look-up table the values of the currents supplied to the sections of the laser corresponding to the measured wavelengths.
Conveniently, the hybridised optoelectronic device further comprises wavelength locking means, and in step b) the wavelength locking means is used to determine when the laser is emitting at one of a plurality of predetermined wavelengths, and in step c) the look-up table is used to store the values of the currents supplied to the sections of the laser corresponding to the predetermined wavelengths.
Preferably, the wavelength measuring means includes a filter transmitting a proportion of an incident beam dependent on the wavelength of the incident beam and the step of using the wavelength measuring means comprises the steps of measuring the power of the incident light; measuring the power of light transmitted by the filter and determining the proportion of the incident beam transmitted to calculate the wavelength of the incident bear
Conveniently, the step of measuring the power of the incident beam comprises the steps of: providing a beam splitter in the path of the incident beam; using the beam splitter to deflect a predetermined proportion of the incident beam out of the waveguide, and measuring the power of the proportion of beam deflected out of the waveguide.
According to an embodiment of the fourth aspect of the invention, there is provided a method of characterising a hybridised optoelectronic device comprising a tunable multi-section laser, having front, gain, phase and back sections; hybridised with an optical filter for transmitting a proportion of power of an incident light beam emitted by the laser, the proportion being dependant on the wavelength of the incident light beam, the method including the steps of: a) applying constant currents to the gain and phase sections such that the leser emits laser radiation; b) applying back and front currents in stepwise increments to the back and front sections respectively; c) measuring power output by the laser to determine values of front and back currents for which the laser emits radiation at wavelengths remote from mode boundaries of the laser; d) measuring the proportion of power transmitted by the filter to measure the wavelength of the emitted radiation; and e) storing in a look-up table the values of front and back current for which the laser emits radiation at wavelengths remote from mode boundaries and the corresponding wavelengths of the radiation.
Preferably, step b) comprises applying sampling currents to determine the position of the mode boundaries.
Conveniently, the step of applying sampling currents comprises the steps of b1) holding the front current at a first front constant and stepping the back current, b2) holding the front current at a second front constant and stepping the back current, b3) holding the back current at a first back constant and stepping the front current, b4) holding the back current at a second back constant and stepping the front current, and b5) stepwise increasing the front current from a third front constant to a fourth front constant while stepwise decreasing the back current from a third back constant to a fourth back constant in order to determine stable middle lines within each super-mode.
Preferably, having determined the stable middle lines, subsequent steps of stepping the back current and/or the front current respectively comprises stepping the respective current through a window of a plurality of incremental values along the stable middle lines and determining for which of the plurality of incremental values the power output is a minimum, and repeatedly incrementing each of the plurality of incremental values and re-determining the current value corresponding to the minimum output power within the window to determine a current value corresponding to a local minimum in the power output.
Advantageously, step e) comprises determining midpoints between the current values corresponding to local minima in the power output to obtain stable middle points of operation of the laser and storing data representative of such stable middle points together with the corresponding wavelength of emitted laser light in the look-up table.
Preferably, operational conditions for operating the frequencies between the stable middle point frequencies are determined by determining and storing in the look-up table the required values of phase current injected into the phase section of the laser.
Conveniently, the required values of phase current are determined by holding the back and front currents constant successively at a first stable point and incrementing the phase current until a frequency of laser emission corresponding to a next stable point is reached and calculating what increments of phase current are required to step from the first stable point to the second stable point in desired frequency increments.
Advantageously, gain currents injected into the gain section of the laser are stored in the look-up table such that the laser can be operated at the same power output at all frequencies.